Ring oscillator circuits can be used to monitor process variations in manufacturing transistors. Historically, ring oscillators have been constructed used complementary metal-oxide semiconductor (CMOS) logic (a combination of n-channel metal-oxide semiconductor (NMOS) transistors and p-channel metal-oxide semiconductor (PMOS)). CMOS ring oscillators may not always be able to accurately distinguish between NMOS and PMOS process variations.
The figures illustrate specific aspects of the circuits. Together with the following description, the figures demonstrate and explain the principles and operation of the circuit. In the drawings, the schematic circuit elements can represent their equivalent structures and connections. The same reference numerals in different drawings represent the same element, and thus their descriptions will not be repeated.